The Sky Palace of Estard (Past)

Go back towards the west, and a short cutscene will play where you'll be summoned to Estard Castle.

When you're ready, head to Estard and explore the town before continuing to the castle. Once inside, head up to the throne room before you start exploring the castle itself.

After you're done, head out towards the east side of the island to get to the Shrine of Mysteries. There's not much you can do here yet, so talk to Kiefer and progress the story to recruit your first party member.